A typical conventional technology capable of detecting the floating of an intermediate tap is disclosed, for example, in patent document 1. According to this conventional technology, a protection circuit for a secondary battery is proposed in which a connection line from the intermediate tap to voltage detection means is connected with a power line via a pull-up resistor or a pull-down resistor and, when the intermediate tap is disconnected, the voltage on the connection line increases to a charge prohibiting voltage, thereby preventing an occurrence of an overvoltage and an overcharge.
However, the above conventional technology has a problem that loss occurs in a steady state due to the pull-up resistor or pull-down resistor. Accordingly, the resistance value of the pull-up resistor or the pull-down resistor needs to be increased to reduce the loss.
On the other hand, the battery pack often includes, in addition to the voltage detection means, a double protection IC or the like for detecting terminal voltages of the respective cells, detecting an abnormality based on a threshold value equal to or higher than that of charge and discharge controller and performing a protection operation. If the double protection IC is provided, charge terminals of the battery pack are connected via uniform internal resistors in the double protection IC. Thus, cell voltages to be measured are divided into voltages equally constituting the total voltage between the charge terminals, which leads to a problem of being unable to detect an actual overcharge state. Specifically, in the case where some of the cells are in an overcharge state (state where abnormality is originally supposed to be detected by the double protection IC) due to a cell balance disorder even if the total voltage is not abnormal in the actual battery, the total voltage is made uniform by voltage dividing resistors in the double protection IC if the intermediate tap is floating. Thus, neither an overcharge detection by the voltage detection means and the charge and discharge controller nor an overcharge detection by the double protection IC functions, wherefore charge is continued even in the overcharge state.
Accordingly, if another circuit such as the double protection IC is provided in parallel with the voltage detection means, it becomes even more difficult to detect the disconnection of the connection lines to the terminals of the respective cells due to the input resistance and capacitance of the other circuit, the internal voltage dividing resistors thereof and the like. Particularly, if the resistance value of the pull-up resistor or the pull-down resistor is increased to reduce the loss as described above, a voltage inputted to the voltage detection means upon the disconnection of the intermediate tap does not largely vary, making detection difficult.
Patent Document 1:
Japanese Unexamined Patent Publication No. H10-150721